In the prior art, a fluid pressure cylinder that is employed in a clamp device or the like that clamps an object to be clamped, such as a workpiece that is to be supplied to machining, has been provided with a cylinder main body, an output member that is fitted in the cylinder main body so as to move freely forward and backward, and a fluid chamber that drives the output member to either advance or retract.
Now, various rod position detection techniques have been implemented in practice for detecting a prescribed position of the output member of the fluid pressure cylinder along its axial direction (this position may be an advanced limit position, a retracted limit position, an intermediate position, or the like).
For example, the clamp device of Patent Document #1 detects the position of a piston rod with a pressure sensor that detects the fluid pressure supplied to a fluid pressure cylinder, and a two position sensor that detects an elevated position and a lowered position of a detection portion of the lower end portion of an actuation rod that projects from a piston member of the fluid pressure cylinder to the exterior.
In the clamp device of Patent Document #2, a mechanism is provided that opens and closes an air passage together with raising and lowering action of an output rod of a fluid pressure cylinder, and thus it is arranged for it to be possible to detect the raised position and the lowered position of the output rod.
In the clamp device of Patent Document #3, a workpiece support stand that receives and holds the subject to be clamped is provided independently. The workpiece support stand includes a pad member in which an air ejection outlet is formed, and an outer barrel member that elastically supports the pad member toward the object to be clamped. Pressurized air is ejected from the air ejection outlet when the pad member is in the projected position, and, when the clamp device is driven for clamping and the pad member is pressed by the object to be clamped and retracts, the air ejection outlet is blocked by the outer barrel member and the pressure of the pressurized air rises, so that it is possible to detect that the clamped state has been established.    Patent Document #1: Japanese Laid Open Pat. Publication 2001-87991;    Patent Document #2: Japanese Laid Open Pat. Publication 2003-305626;    Patent Document #3: Japanese Laid Open Pat. Publication 2009-125821.
With the clamp device of Patent Document #1, since the actuation rod is projected to the exterior from the piston member of the fluid pressure cylinder, and the raised position and the lowered position of the detection portion provided on the lower end portion of the actuation rod are detected by the two position sensor, accordingly it is necessary to provide a detection space below the fluid pressure cylinder to allow shifting of the detection portion and for installation of the position sensor, so that there is the problem that the clamp device (i.e. the fluid pressure cylinder) is increased in size.
With the clamp device of Patent Document #2, the mechanism that detects the raised position and the lowered position of the output rod is provided outside the clamp main body. Due to this, in a similar manner to the case with the clamp device of Patent Document #1, it is not possible to make the clamp device compact, since it is necessary to provide a detection space external to the clamp main body. Moreover, since in this structure a detection element that opens and closes the air passage is shifted so as to slide freely with respect to a detection orifice, there is a fear that, when used over the long term, the air passage may become blocked and the performance may decrease.
And since, during the unclamped state, the air ejection outlet of the workpiece support stand of the clamp device of Patent Document #3 opens at a location in the neighborhood of the clamp device and of the object to be clamped, there is a fear that swarf resulting from machining or coolant (i.e. cutting liquid) may enter into the air ejection outlet and block it.
The objects of the present invention include: to provide a fluid pressure cylinder and clamp device that can be made more compact while being capable of reliably detecting, on the basis of pressure change of air pressure in an air passage within a cylinder main body, that an output member has arrived at a prescribed position; and to provide a fluid pressure cylinder and clamp device that provide enhanced reliability and durability for detection of a prescribed position of an output member.